Archive for April 12, 2011

It’s time for another round of T-SQL Tuesday and this round of the revolving blog party is being hosted by Matt Velic [Blog | Twitter].

APPLY – That is the topic for this month’s T-SQL Tuesday! The APPLY operator was added to the T-SQL repertoire and which has resulted in lesser use of cursors for a large number of diagnostic scripts that CSS uses to collect data while working on SQL Performance issues. In this blog, I shall share a few examples of such queries that we use to collect data while working on SQL Performance cases.

TOP Query Plan Statistics

The following query gives you a list of the SQL batches/procedures with their CPU usage, Query/Batch duration and Physical Reads rank. This query helps identify the TOP CPU/Duration/Read consuming queries by making use of system DMVs. The output below is useful for the following reasons:

1. I get the usecount of the procedure/batch and if this batch is called multiple times and the use count of a Compiled Proc cached object is only 1, then the plan is not being re-used. This now tells me that I need to look at reasons behind inability of plan re-use.

2. I get the total and average resource usage statistics for each of the queries listed in the output.

3. A quick glance at the output gives me an idea of the most expensive queries on the instance w.r.t. reads or/and CPU and/or query duration.

This is a query which can help you identify queries which have the same query plan hash

SQL Server Books Online topic “Finding and Tuning Similar Queries by Using Query and Query Plan Hashes” has more information on this topic. The query hash feature was added in SQL Server 2008 which made it easier to troubleshooting performance issues caused by ad-hoc queries which differed in just literal values. RML Utilities does a similar task by creating query hash but now if you are troubleshooting on the server, you can do this using DMVs without having to capture a profiler trace.

The right operand supplied to the Apply operator is a function of one or more column values that are present in the left operand. So basically, the right operand is a table-valued expression of which is evaluated once for each row that appears in the left operand. The Cross Apply and Outer Apply are the two flavors of the Apply operator. So if I wanted to simulate an Apply Operation without the Operator itself, it would require the use of temporary tables or table variables.

I use the APPLY operator a lot while parsing XML data like Process Monitor traces or XML query plans which make life a lot easier and saves me from writing a huge bunch of T-SQL code.